1. Field of the Invention
The present invention broadly relates to the field of lithographic systems; and more particularly, to a photolithographic method of maintaining optimum tool performance when photoresist batches are changed on a photolithography cluster. A predictive method is used to compensate for batch sensitivity changes which inevitably occur during resist batch changeover. The compensation is applied to a nominal exposure condition to maintain proper image size.
2. Background of the Invention
In the field of integrated circuits (ICs), photolithography is used to transfer patterns, i.e. images, from a mask, i.e., reticle, containing circuit-design information to thin films on the surface of a substrate, e.g. Si wafer. The pattern transfer is accomplished with a photoresist (e.g., an ultraviolet light-sensitive organic polymer). In a typical image transfer process, a substrate that is coated with a photoresist is illuminated through a mask and the mask pattern is transferred to the photoresist by chemical developers. Further pattern transfer is accomplished using a chemical etchant. One measure of photoresist is its sensitivity (photospeed) which represents the amount of energy required to expose a semiconductor image to a given size.
When batches of photoresist are changed, there is often a detectable difference in sensitivity between the old batch and the new batch. This difference may be due to ambient temperature, e.g., room temperature, aging. The room temperature aging problem may be exacerbated by the fact that new batches are often kept refrigerated. Thus, when a change from the old to the new is made there exists a sensitivity difference between the batch at room temperature vs. the refrigerated batch. Other differences may result from today""s complex formulations of photoresist. The complexity of these formulations makes it more difficult to control photospeed. Again, this lack of control dictates the need for a predictive exposure condition adjustment means based on the resultant batch sensitivity differences.
FIG. 1 illustrates a photolithographic processing (fab) environment comprising a mask 110, a stepper device 120 with lens 130 through which the exposure energy 140 is focused on a wafer 150 coated with a photoresist 160. The exposed photoresist results in a resist image 170. The resist image may then be developed on the wafer, resulting in a developed image 180.
It is well known in the field of photolithography that the maintenance or proper image size during a batch change is a problem that must be addressed. Often when a new batch is introduced into the system, old historical data is invalidated, requiring the re-establishment of the image size/exposure condition baseline. As a result, elevated rework and decreased product throughput occur. The related art has not been shown to successfully address this issue in the manner of the current invention wherein batch-to-batch variations in resist sensitivity are used for exposure dose correction.
A related art technique for a projection exposure method is described in Suzuki (U.S. Pat. No. 6,235,438 B1) which is directed to a lithography tool adjustment method wherein an image formation is corrected. Light intensity is changed in this technique, however this process does not include measurement of any of the photoresist parameters as intensity adjustment input.
Another related art method is described in Research Disclosure (RD) (BU886-0412) which generally discusses the need for real time control of photo exposure dose. The graph of the RD disclosure reveals a relationship between image size and time delay from a pre-exposure activation treatment to exposure. The RD disclosure however, does not set forth a method of exposure control based on changing the photoresist batch.
A further related method disclosed in Mack (U.S. Pat. No. 5,363,171), involves a method for in situ photoresist measurements and exposure control. The measurements are used in a feedback loop to control the exposure dose, i.e. the exposure is turned off after predetermined absorption and reflectivity data are measured. The method however, does not address the need to monitor the effects of a change in photoresist batch characteristics.
Another related art technique described in Marchman (U.S. Pat. No. 5,656,182) utilizes feedback control, however, does not address maintenance of proper image size by computing a batch factor as a function of historical and current photoresist batch properties. Rather, it merely performs stage position control as a function of the latent image produced in the substrate.
While it is well known in the art that using an active feedback loop in which image size measurements from a current lot are used to adjust the dosing of a future lot, this current technique reacts too slowly when resist sensitivity changes. The dose adjustment, using this prior art technique will tend to lag behind the dose required for the changing resist sensitivity. Additionally, once the resist sensitivity has changed, it invalidates all of the old historical data, requiring the re-establishment of the image size/dose baseline. The result is elevated rework and decreased product throughput.
Thus, there remains a need for a predictive method which can accommodate both the transition period during resist batch changeover, as well as a method which can make the nominally invalid old data usable again.
It is an object of the present invention to provide a photolithographic system and method that is capable of adjusting exposure condition levels to maintain proper image size when an old photoresist batch is either mixed with a new batch, or completely replaced by the new batch.
It is another object of the present invention to provide a system and method which avoids rework and cycle time penalties encountered during significant batch sensitivity changes.
It is a further object of the present invention to provide a system and method that decreases engineering monitoring required to re-stabilize a process. The process to be re-stabilized is not limited to photolithographic processes, but may be any process in which a nominal process result is desired while using materials which may deviate from a standard condition.
Yet another object of the present invention is to provide a photolithographic system and method that keeps product more tightly centered in a specification region, rather than permitting substantial image size deviations.
These and other objects and advantages can be obtained in the present invention by introducing a method for calculating a batch factor that is a dose ratio for each reticle. The batch factor may then be used to predict a xe2x80x9cmixture-averagexe2x80x9d sensitivity, and along with historical data, reliably predict proper dosages during the batch transition period.
The basic consideration of this invention is the development of a batch factor (usually a ratio of current to previous performance) to quantify the change to the component and normalize the data to enable other variables to counter the deviation based on previously existing data in order to attain the desired result. The level of detail of the batch factor depends on how critical an accurate result is to the process, if the system is not tightly controlled the batch factor can be approximated; however, if the final result must meet tight tolerances the derivation of a batch factor may become much more detailed and painstaking based on the system in question.
Specifically, in one aspect of the present invention, a method provides better exposure dose control as a function of resist sensitivity changes, for maintenance of proper image size by performing the steps of calculating a baseline exposure condition, i.e., reference dose; as the exposure condition moves, continuously calculating a batch factor as a ratio of an actual dose number to the reference dose; and using the batch factor to adjust historical data.
In another embodiment, the batch factor may be used to adjust both historical data before a batch changeover and more recent historical data upon detection that a new batch of resist is loaded to a tool. In this embodiment, detection of a new resist load is accomplished by the setting and polling of a software batch change flag. Interim batch factors are used as normalization constants thereby creating a xe2x80x9cwalking batch factorxe2x80x9d.
In another embodiment, the system and method comprises utilizing a wafer pass counter to track how many shots, i.e., applications of resist, have been applied from the dispense system; using the volume of new resist entering the system in conjunction with known mixing characteristics of the dispense system to calculate xe2x80x9c% new materialxe2x80x9d in the system; and, using the batch factor and % new material parameter to calculate the batch factor when complete batch changeover has occurred. When a next lot is run, the new resist shot count is used to calculate a new % new material factor; this is used with the current steady state batch factor to calculate a projected new batch factor based on modeled data to run the new lot. The current steady state batch factor is determined as follows: after a fixed time or after a fixed number of shots have been dispensed from the wafer, there is calculated a steady state batch factor for the resist representing the behavior of the resist before it starts to degrade; an xe2x80x9caging factorxe2x80x9d is applied to the resist sensitivity and, after a known amount of time at room temperature, the aging factor is calculated and multiplied with the current steady state batch factor.
Advantageously, the system and method of this invention are applicable for any application involving lithography, e.g., photolithography, including g-line, i-line, DUV (248 nm), 193 nm, EUV, X-ray, and e-beam exposures or even other microelectronic uses including etch, Chemical Mechanical Polish, Chemical Vapor Deposition, etc. The concept of a batch factor implementing the use of prior data is applicable not only to semiconductor wafer fabricators, but to any process that uses materials which may deviate from a standard condition, thus affecting the outcome of the process. Some example of non-microelectronic uses include flat screen displays, food, pharmaceutical, and chemical production.